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I usually use Calibre, it's available for a variety of platforms: https://calibre-ebook.com/download

This is my current tool: EPUBReader, an add-on for Firefox: https://addons.mozilla.org/firefox/addon/epubreader/

It's my goto PDF and epub reader.

You could have mentioned it is Windows only. Thanks for suggesting it though.

You can upload to Google Play Books. https://play.google.com/books

It works fine on my iphone (iBooks - installed by default).

They asked about readers for desktop, specifically Windows. Pretty sure iBooks is not relevant.

iBooks run on the desktop as well and syncs back and forth.